Medical catheters having a balloon mounted thereon are useful in a variety of medical procedures. Balloon catheters may be used to widen a vessel into which the catheter is inserted by dilating the blocked vessel, such as in an angioplasty procedure. Balloon catheters may also be used to expand and/or seat a medical device such as a stent or graft at a desired position within a body lumen. In all of these applications, fluid under pressure is supplied to the balloon through an inflation lumen in the catheter, thereby expanding the balloon.
It is essential in the manufacture of balloon catheters to properly seal the balloon to the catheter. The seal must be able to withstand the high pressures to which it is subjected on inflation of the balloon. A poor seal may result in leakage of inflation fluid and inability to achieve the desired pressure or even rapid loss of pressure and deflation of the balloon.
Such seals may be formed using a fusion-based thermal bonding method such as disclosed in U.S. Pat. No. 5,501,759 to Forman involves the use of a beam of laser radiation at a wavelength selected to at least approximately match a wavelength of maximum spectral absorption of the polymeric materials forming the balloon member and body. The polymeric materials are melted by the radiation and then allowed to cool and solidify to form a fusion bond between the catheter tube and the balloon. In order to bond the balloon about its entire circumference to the catheter tube, the catheter tube may be rotated relative to the laser beam or the laser beam may be rotated relative to the catheter tube. Thus the bonds that are formed in this way are limited to bonds that are symmetric about the axis of rotation. The laser beam is typically focused onto the polymeric materials by one or more lenses or other optical arrangements.
The use of a focused laser beam in a thermal bonding process presents a number of limitations because the energy density that is applied to the polymeric materials is increased. For example, three-dimensional alignment of the beam and the catheter is critical because a focused beam has a relatively small spot size and a limited depth of focus. Alignment tolerances become even more stringent because the laser beam generally has a gaussian or near Gaussian distribution across its width. Such a distribution gives rise to an energy density that is greatest at the center of the beam and which decreases toward the beam edge. Thus, in order to maintain a uniform power level across the bond site, not only must the bond site be aligned with the focused beam, but it also must be aligned within the focused beam at the precise location at which the desired energy density is to be achieved.
In addition to more severe alignment requirements, the use of a focused beam also requires a higher degree of power stability than with the use of an unfocused beam. This is because a factor of two decrease in the diameter of the focused beam size gives rise to a factor of four increase in power density. As a result, relatively small power fluctuations that occur when the beam is generated become magnified when the beam is focused. For example, if an unfocused beam delivers 1 watt of power with a 4 mm beam diameter, the power density would be approximately 0.08 w/mm2. However, if the beam is focused to a 1 mm diameter, the power density increases to about 1.3 w/mm2. Thus a small percentage change in the power of the beam when it is generated can lead to large power fluctuations after the beam is focused. Delivery of a precisely controlled energy density is particularly important when the materials to be bonded are polymeric materials that have relatively low energies of transformation and which can undergo a transition from a bonding state to a burned state very easily.
Of course, for laser welding a certain average power is required to melt and bond or weld polymers. If a larger diameter beam is employed the laser power needs to be increased commensurately to obtain the same average power needed to form the bond. Thus, regardless of the beam diameter a 10% power fluctuation is still a 10% change about the optimum power and adds uncertainty to the bond conditions.
If the catheter tube is rotated, rotation speeds of 400 rpm or higher are necessary to ensure even heating of the catheter tube and balloon material. Care must be taken, however, to avoid damaging the catheter during rotation. Moreover, the catheter will have a tendency to wobble as it rotates, which may cause the bond site to move in and out of the focused beam. While this problem can be reduced by securing the catheter to a fixture at multiple locations along its length, this adds to the complexity of the fixture. Finally, the materials to be bonded must remain in intimate contact as they are rotated, thus requiring some means for preventing relative motion between them. If, for example, a heat shrink material is used to form a tight joint between the materials, the size of the heat shrink material must be reduced before the bonding process is begun. That is, the size of the heat shrink cannot be reduced by the bonding process itself, but must be performed beforehand, thus necessitating an additional preparatory step before the bonding process.
Instead of rotating the catheter the laser beam may be rotated via the use of mirrors and focusing lenses. Alignment can be difficult to achieve and maintain, however, because of vibration from moving parts. The process is slow because of the time involved in loading and unloading the catheter and for waiting for the rotational beam to reach the desired speed. Moreover, such an arrangement can be expensive to build.